An oil supply type compressor is known that uses oil to produce compressed air for the principal purpose of cooling of air in the compressor, sealing-off of a compressing chamber and lubrication of the compressor and/or the like.
Compressed air compressed to a predetermined pressure inside a compressor body of the oil supply type compressor is mixed with lubricating oil and discharged. Then, after the compressed air and the lubricating oil are separated from each other by an oil separation mechanism (primary separation) and an oil separator (secondary separation) which are located in an oil tank forming a part of an oil separation device, the compressed air is delivered outside from the compressor to be supplied to a use site of the user.
The separation of lubricating oil from compressed air is often performed in two steps, primary separation and secondary separation. In many instances, the primary separation uses centrifugal force on or collision of the lubricating oil in the oil tank to separate the lubricating oil from the compressed air, and the secondary separation uses a filtering element to separate the lubricating oil from the compressed air.
In contrast, the lubricating oil thus separated is temporarily stored in the oil tank. Then, the lubricating oil is cooled by a cooler and then re-supplied into the compressor body for circulation.
When the amount of air usage of the user is reduced to reach a predetermined pressure (specification pressure), capacity control of the compressor is performed to stop the supply of compressed air. The capacity control includes the following controls to achieve a reduction in power of the oil supply type compressor (reduction in power consumption).
(1) An intake throttle valve on the inlet side of the compressor is closed, and the compressor body is shut down by stopping the motor. At this stage, the compressed air after flowing through the oil separator is released through an air release path into the atmosphere to reduce the pressure inside the oil separator and the oil tank to atmospheric pressure or nearly to atmospheric pressure. Then, upon reduction in the compressed air pressure on the user side to a predetermined pressure, the operation of the compressor body is re-started, the intake throttle valve is open, and the air release path is closed for the resumption of compression.
In this regard, for restarting the operation of the compressor body, a shorter time from the shutdown of the compressor body to a restart causes startup stall due to residual pressure inside the oil separator at restart because the pressure inside the oil separator (likewise inside the oil tank) does not reduce to reach the atmospheric pressure. Since a predetermined time period is required for reducing the pressure inside the oil separator, the limited time until a restart is enabled is provided in order to prevent the startup stall from taking place due to the residual pressure inside the oil separator at restart. The capacity control will be hereinafter referred to as “automatic stopping control”.
(2) Without a motor shutdown, while the operation of the compressor body is maintained, the intake throttle valve on the inlet side of the compressor is closed, and the compressed air after passing through the oil separator is released through the air release path into the atmosphere to reduce the operation pressure of the compressor (outlet pressure). Then, upon reduction in the compressed air pressure on the user side to a predetermined pressure, the intake throttle valve is open, and the air release path is closed to resume the supply of the compressed air to the user. The capacity control will be hereinafter referred to as “no-load operation control”.
The compressor body is shut down in the above-described automatic stopping control (1). On this account, the automatic stopping control produces greater effects of reducing the compressor power than the no-load operation control (2). However, if the amount of compressed air consumed by the user is largely varied (large load changes), then the operation of the compressor is repeatedly stopped for a short time, resulting in an increase in burden on the motor driving the compressor body. When the limited time until the restart is enabled is provided, the amount of compressed air supplied to the user may possibly not be adequate. Given these circumstances, when the amount of compressed air consumed by the user is greatly varied and the motor is stopped at high frequency, switching to the no-load operation control (2) is typically made.
In the capacity controls (1) and (2), the pressure in the oil separation device including the oil tank and the oil separator is reduced below the pressure on the user side (pressure in a reservoir for holding the compressed air), so that a check valve is installed downstream of the oil separator in order to prevent backflow of the compressed air from the user side toward the oil separator.
In each capacity control (1), (2), the compressed air after passing through the oil separator is released through the air release circuit into the atmosphere. The air release circuit includes air-release piping connecting the downstream end of the oil separator and the atmosphere to each other, in which a pressure of the compressed air on the user side is detected, and when the pressure reaches a maximum value, the solenoid valve installed in the air release piping is opened to release the compressed air passing through the oil separator into the atmosphere.
In any of the automatic stopping control and the no-load operation control, the air release circuit is typically a single circuit shared between them. The adjustment of time required for air release is made by using an orifice and/or the like provided in the air release circuit to adjust the flow rate of released air.
In the capacity control, it is desired to shorten, as much as possible, the length of time required for a reduction of the pressure inside the oil separator to the atmospheric pressure (pressure drop time period). The reasons for this is that, in the automatic stopping control, the limited time to the subsequent restart can be shortened by shortening the pressure drop time period, so that a swift supply of the compressed air is enabled in response to the load changes on the user side. Furthermore, in the no-load operation control, shortening the pressure drop time period enables a drop in pressure on the outlet end of the compressor body to a lower level, resulting in a reduction in power during the process of pressure drop.
However, a quick drop in pressure inside the oil tank to around the atmospheric pressure causes occurrence of so-called foaming resulting from expansion of bubbles concentrated in the lubricating oil to generate larger bubbles.
The shorter the time of drop in pressure inside the oil tank, the faster the foaming grows. If the pressure drops sharply, a cluster of bubbles may possibly move upward in the tank and then through the oil separator to flow to the user.
To avoid this, Patent Literature 1 (Japanese Patent Application Laid-Open No. H05-296174) describes an oil supply type compressor configured to shorten the pressure drop time period and prevent the foaming.